Method of working aluminum-magnesium alloys to confer satisfactory stress corrosion properties

ABSTRACT

Providing an aluminum-magnesium alloy containing from 5.0 to 10.0 percent magnesium, heating said alloy to a temperature range of 650* to 800* F. for 1 to 16 hrs., cooling said alloy. heating said alloy to a temperature range from 225* to 375* F. for 15 mins. to 24 hrs., cooling said alloy to ambient temperature, cold reducing said alloy to 5.0 to 95.0 percent reduction, heating said alloy to a temperature range to 225* to 375* F. for 15 mins. to 24 hrs., and cooling said alloy.

United States Patent Inventor Francis P. Ford Hamden, Conn. Appl. No.814,631 I Filed Apr. 9, 1969 Patented Nov. 1, 1971 Assignee OlinMathleson Chemical Corporation METHOD OF WORKING ALUMINUM- MAGNESIUMALLOYS TO CONFER SATISFACTORY STRESS CORROSION PROPERTIES Claims, 1Drawing Fig.

U.S. Cl 148/1 1.5 A, 148/ l 2.7 Int. Cl C22f l/04 Field ofSearch [48/]1.5, 12.7

[56] References Cited UNITED STATES PATENTS 3,346,371 /1967 Jagaciakl48/l1.5 3,346,372 10/1967 Jagaciak l48/l 1.5

Primary Examiner-L. Dewayne Rutledge Assistant Examiner-W. W. StallardAnorneys Robert H. Bachman, Richard S. Strickler, Donald R. Motsko andThomas P. ODay EFFECT OF DlgPLEXSHB/L/ZMF TREATMENTOF WPE COLDROLL 12707' X dfiDl/R-f COLDROLL 1 27D! XII'DURS ONSTRESS CORROSDN PROPERTIESOFAL- 73m -Q2ZCR- aooaza ALLOY,

HEAf/NG 8 COOL/N6 RATES/N HEAT TREATMENTS JJ F/I-DUI? so 7 a g l u k g;8 40 kt Q 1 o 1 a s 5 a a a s f: s fi E E '50 'tr 5 l w b g g Q 0 u u ao 55 8 f 7 g 8 5 E '1' '0 40 a 190 s Q E. t I 3 kn 8 e t --Z0 0 Q 50 7502 l 8 09 a0 cowmr/amr can not 0 50 do STAB/HI TREATMENT. 0 I0 I a0 40 STRLSS CORROSION LIFE -HOURS w w w M m H n a H M INVENTOR ATTORNEY am wFRANCIS I? FORD kco/vmv IOML com ROLL sma/uzz TREATMENT.

S TRESS- CORROSION. LIFE -HOUR$ PATENTED uuvz l97l EFFECT OF DQPLEXSHE/LUNG TREATMENTOF TYPE COLD ROLL f 270?" x 4HOUR COLD ROLL f 270/ x4l-DURS 0N STRESS CORROSDN PROPERTIES OF AL- 776016 0.2% R- 0.00828ALLOY, v HEAT/1V6 & cooulva RATES w HEAT TREATMENIS'. 33F/ HOUR OVERALLTEMPERS I 11-80 MNQ\QSPW QZOQMM MQOKMQ EORUDQHQ QQQU m METHOD OF WORKINGALUMINUM-MAGNESIUM ALLOYS TO CONFER SATISFACTORY STRESS CORROSIONPROPERTIES The present invention relates to a new and improved method ofproducing aluminum-base alloys containing magnesium. More particularly,the present invention resides in aluminum-base alloys containing fromabout 5.0 to about percent magnesium and characterized by improvedstress corrosion resistance due to the improved partitioning of amagnesium-rich phase between the grain boundaries and the grainmatrixes, i.e., to a different volume ration thereof.

The advantages to be derived from alloying magnesium with aluminum-basealloys were recognized very early in the development of aluminumtechnology. Consequently, the aluminum-magnesium series of alloys is oneof the oldest used commercially.

It is well known, however, that magnesium in aluminumbase alloys ifpresent in an amount more than about 3 percent sensitizes the alloy tostress corrosion. Retention of magnesium in solid solution is readilyachieved by annealing the alloy at a temperature above the solvustemperature and cooling at a rate rapid enough to prevent precipitationof a magnesiumrich second phase. The alloy may then be cold worked tofinal gauge. However, due to natural aging at ambient temperaturemagnesium retained in solid solution in excess of about 5.0 percent bythe rapid cool tends to precipitate preferentially in the grainboundaries as an aluminum-magnesium intermetallic .compound thussensitizing the alloy to stress corrosion.

Furthermore, the mechanical properties of the cold-worked alloy tend todegrade during service due to thermal recovery, which also occurs at ornear ambient temperature.

In order to prevent degradation of the mechanical properties, it isnecessary to stabilize the alloy after the final coldworking step at atemperature somewhat above that which it will be subjected to inservice. Thus, the alloy will not undergo subsequent change ofmechanical properties at temperatures significantly below thestabilizing temperature.

As is well known, some improvement in resistance to stress corrosion maybe obtained if the alloy is slowly cooled, i.e., less than 500 F. perhr., after the finalanneal prior to cold working to promoteheterogeneous nucleation of the equilibrium magnesium-rich phase in thegrain matrix as well as in the grain boundaries, rather than solely orpredominately in the grain boundaries as will occur upon aging of thealloy. The stabilizing treatment, however, in those alloys containingmore than 5.0 percent magnesium causes additional heterogeneousnucleation of the equilibrium magnesium-rich beta phase, or a metastablebeta modification, in the grain boundaries and, should the alloy behighly cold worked, at points of threedimensional disregistry indeformation bands.

Precipitation of the aforementioned magnesium-rich phase preferentiallyin the boundaries causes susceptibility to stress corrosion whichincreases with increasing magnesium content. As a result, the magnesiumcontent of the aluminum-magnesium alloys is generally limited to about5.5 percent magnesium, thus precluding favorable strength properties atmagnesium contents in excess of 5.5 percent.

Accordingly, it is a principal object of the present invention toprovide a new and improved process whereby stress corrosionsusceptibility in the aluminum-magnesium alloys is substantiallyreduced, and the alloys produced thereby.

lt is-a still further object of the present invention to provide aconvenient and expeditious process as aforesaid at reasonable cost.

Further objects and advantages of the present invention will appearhereinafter.

The process of the present invention comprises; (A) providing analuminum-magnesium alloy containing from about 5.0 to about l0.0 percentmagnesium, balance essentially aluminum, (B) heating said alloy to atemperature range of from about 650 to 800 F. for from about 1 to 16hrs., wherein the rate of heating from about 350 F. is not greater thanabout 50 F. per hr., (C) cooling said alloy wherein the rate of saidcooling is not greater than about 50 F. per hr. to about 350 F., (D)prestabilizing said alloy in a temperature range of about 225 to about375 F., for about 15 min. to 24 hr., (5) cooling said alloy to ambienttemperature, (F) cold reducing said alloy to about 5.0 to about 95.0percent reduction, (G) stabilizing said alloy in a temperature range ofabout 225 to about 375 F. for about 15 min. to 24 hr., and; (H) coolingsaid alloy.

Normally, the heating up and cooling-down rate to and from theprestabilization and stabilization treatments is about l5 to 50 F. perhour in commercial practice. The present invention however is notrestricted in this respect and higher or lower heating rates may bereadily employed. It is to be further noted that the alloy preferably isprovided in cold-reduced form in step A.

Although the heating range for the thermal stabilization temperature isgenerally in the order of 275 to 300 F. the present invention isapplicable to other temperatures as may be dictated by conventional millpractices,'i.e., 225 to 375+ F.

In an alternative embodiment of the present invention an additional coldreduction may be provided prior to step D. Thus, the alloy may be coldreduced from about 5.0 to about 95.0 percent reduction prior to theprestabilization treatment.

The drawing shows that for the overall tempers of H-34, H- 36 and "-38corresponding to 40 percent, 60 percent and percent cold reductionrespectively, that the stress corrosion properties are greatly improvedfrom the unsatisfactory values obtained when a conventional full annealfollowed by a cold roll and stabilization is employed.

The data in the drawing indicates that in general the improvement instress corrosion resistance increases, in the case of metal provided incold-reduced form as in the alternative embodiment, with the amount ofthe final cold roll, i.e., step F, for any given temper. It is also seenthat optimum increase in stress corrosion resistance is achieved inaccordance with the preferred embodiment, omitting the cold roll justprior to the prestabilization treatment. In this illustration theprestabilization and stabilization treatments were both conducted at atemperature range of 270 F. for 4 hr., and with the heating and coolingrates at about 33 F. per hour to simulate actual commercial practicewherein coil annealing furnaces are employed.

Naturally other elements may be present in the aluminummagnesium alloysas alloying additions or impurities. Common alloying additions mayinclude but are not limited to the following: boron in an amount from0.001 to 0.35 percent; chromium in an amount from 0.05 to 0.3 percent;indium in an amount from 0.002 to 0.80 percent; gallium in an amountfrom 0.0l to 0.50 percent; cadmium in an amount from 0.03 to 0.50percent; thorium in an amount from 0.005 to 0.350 percent; misch metalin an amount from 0.005 to 0.30 percent; tellurium in an amount from0.005 to 0.30 percent; lithium in an amount from 0.0l to 0.80 percent;germanium in an amount from 0.0l to 0.55 percent; cobalt in an amountfrom 0.10 to 0.80 percent; copper in an amount from 0. l0 to 0.60percent.

Naturally small amounts of elements may also be present in thealuminum-magnesium alloys as impurities. Impurities may include but arenot limited to the following; iron up to 0.50 percent; silicon up to0.50 percent; copper up to 0.25 percent; manganese up to 0.35 percent;zinc up to 0.2 percent; titanium up to 0.l5 percent; beryllium up to0.02 percent; and others in total up to 0.2 percent.

The present invention is of considerable commercial importance inrelation to high magnesium containing alloys. As shown in the drawingconventional fabrication of such wrought alloys gives unsatisfactorystress corrosion resistance while the method of the present inventionprovides for a satisfactory stress corrosion life with an adequatesafety margm.

The present invention will be more readily apparent from anconsideration of the following illustrative examples.

EXAMPLE I An alloy having the following composition was prepared from acharge of commercial purity aluminum, master alloys of iron-aluminum,chromium-aluminum, beryllium-aluminum, titanium-aluminum and the otheralloying additions in elemental form. The alloy was cast in the form of45Xl6 l20- inch ingots.

TABLE I Mg Cr Fe Si Cu Mn Zn Ti Be B Alloy. 7.47 0.19 0.26 0.10 0.060.01 0.01 0.002 0.005 0.007

EXAMPLE ll EXAMPLE Ill This example shows the results obtained inaccordance with the present invention and shown in the drawing.

Following hot rolling to the intermediate gauges the alloys were coldrolled to 0.060 inch, employing various amounts of reduction prior tothe prestabilization and stabilization treat-v ments. It may be seenhowever that all the alloys in the 40, 60, and 80 percent cold-reducedcondition obtained the greatest resistance to stress corrosion when thealloys were cold rolled directly from the intermediate gauges to thefinal gauge of 0.060 inch prior to the stabilization treatment, i.e.,when the additional cold roll prior to the prestabilization treatmentwas not provided, in accordance with the preferred embodiment of thepresent invention. It is seen that significant improvement was stillobtained, however, with the prior cold roll at various reductions whencontrasted with the conventional cold roll and stabilizing treatmentalthough the improvement was not as great as in the aforementionedpreferred embodiment.

EXAMPLE IV Stress corrosion testing of the alloy of example I after thetreatments of examples ll and Ill was conducted in the followingaccelerated manner:

Samples 0.060X2.0X0.25 inch were stressed at 80 percent of their yieldstrength in a 6 percent solution of NaCl +0.005M' NaHCO,. An anodiccurrent of 11 ma./sq. in. was applied via platinum gauze cathode. Afailure time of 13 hr. in the accelerated tests corresponds to a failuretime for preformed U-band specimens in a marine environment of greaterthan 3 years, a limit which normally signifies a stress corrosionresistant condition.

This invention may be embodied in other forms or carried out in otherways without departing from the spirit or essential characteristicsthereof. The present embodiment is therefore to be considered as in allrespects illustrative and not restrictive, the scope of the inventionbeing indicated by the appended claims, and all changes which comewithin the meaning and range of equivalency are intended to be embracedtherein.

What is claimed is:

A. Providing an aluminum-magnesium alloy containing from 5.0 tol0.0percent magnesium.

B. heating said alloy to a temperature range of 650 to 800 F. for 1-6hr., the rate of said heating not greater than 50 F. per hr. from 350 F.

C. cooling said alloy, the rate of said cooling not greater than 50 F.per hr. to 350 F D. heating said alloy to a temperature range of from225 to 375 F. for 15 min. to 24 hr.,

E. cooling said alloy to ambient temperature,

F. cold reducing said alloy from 5.0 to 95.0 percent reduction,

G. heating said alloy to a temperature range of from 225 to 375 F. for15 min. to 24 hr.,

H. cooling said alloy.

2. A process according to claim 1 wherein said alloy is cold reducedfrom 5.0 to 95.0 percent reduction following step C and prior to step D.

3. A process accordingto claim 1 wherein said alloy contains an alloyingsubstituent selected from the group consisting of 0.001 to 0.350percentboron, 0.05 to 0.3 percent chromium, 0.002 to 0.80 percent indium, 0.01to 0.50 percent gallium, 0.03 to 0.50 percent cadmium, 0.005 to 0.350percent thorium, 0.005 to 0.30 percent misch metal, 0.005 to 0.30percent tellurium, 0.01 to 0.80 percent lithium, 0.01 to 0.55 percentgermanium, 0.10 to 0.80 percent cobalt, 0.10 to 0.60 percent copper andmixtures thereof.

4. A process according to claim I wherein said alloy contains as animpurity an element from the group consisting of iron up to 0.50percent, silicon up to 0.50 percent, copper up to 0.25 percent,manganese up to 0.35 percent, zinc up to 0.2 percent, titanium up to0.15 percent, beryllium up to 0.02 percent, total all others up to 0.2percent, and mixtures thereof.

5. A process according to claim 1 wherein said alloy contains 6.0 to 8.0percent magnesium, 0.001 to 0.350 percent boron, 0.05 to 0.3 percentchromium and as impurities iron up to 0.5 percent, silicon up to 0.5percent, copper up to 0.25 percent, manganese up to 0.35 percent, zincup to 0.2 percent. titanium up to 0.25 percent, beryllium up to 0.02percent, total all others up to 0.2 percent.

* i i i i UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PatentNo. 3, 617,395 Dated November 2, 1971 ln fl Francis P. Ford It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

In the heading, line 5, after Nov. delete "l" and insert 2 Golunm 2,line 19, after 375 delete and insert Column 2, line 47, after "5 insertO Column 3, line 21, after "produce" insert alloys Column 4, line 17,delete "6" and insert l6 Signed and sealed this 18th day of April 1972.

(SEAL) A ttest:

EDWARD I LFLETCHIUR ,JR ROBERT GOT'ISCHALK Attesting OfficerCommissioner of Pfitents RM PO'WSO H069) USCOMM-DC scam-s69 n U 5GOVERNMENT FHINING OF'ICE 1969 0-356-334

2. A process according to claim 1 wherein said alloy is cold reducedfrom 5.0 to 95.0 percent reduction following step C and prior to step D.3. A process according to claim 1 wherein said alloy contains analloying substituent selected from the group consisting of 0.001 to0.350percent boron, 0.05 to 0.3 percent chromium, 0.002 to 0.80 percentindium, 0.01 to 0.50 percent gallium, 0.03 to 0.50 percent cadmium,0.005 to 0.350 percent thorium, 0.005 to 0.30 percent misch metal, 0.005to 0.30 percent tellurium, 0.01 to 0.80 percent lithium, 0.01 to 0.55percent germanium, 0.10 to 0.80 percent cobalt, 0.10 to 0.60 percentcopper and mixtures thereof.
 4. A process according to claim 1 whereinsaid alloy contains as an impurity an element from the group consistingof iron up to 0.50 percent, silicon up to 0.50 percent, copper up to0.25 percent, manganese up to 0.35 percent, zinc up to 0.2 percent,titanium up to 0.15 percent, beryllium up to 0.02 percent, total allothers up to 0.2 percent, and mixtures thereof.
 5. A process accordingto claim 1 wherein said alloy contains 6.0 to 8.0 percent magnesium,0.001 to 0.350 percent boron, 0.05 to 0.3 percent chromium and asimpurities iron up to 0.5 percent, silicon up to 0.5 percent, copper upto 0.25 percent, manganese up to 0.35 percent, zinc up to 0.2 percent,titanium up to 0.25 percent, beryllium up to 0.02 percent, total allothers up to 0.2 percent.